Time-dependent correlation functions of (unstable) particles undergoin
g biased or unbiased diffusion, coagulation and annihilation are studi
ed. This is achieved by similarity transformations between different s
tochastic models and between stochastic and soluble non-stochastic mod
els. For special cases we obtain exact results which are in good agree
ment with experiments on one-dimensional annihilation-coagulation proc
esses.